This invention relates to solder deposition methods and more specifically to a method for depositing and segregating a thick pad of solder upon a relatively thin circuit trace.
Current technology, such as VHSIC (Very High Speed Integrated Circuit) technology, permits fabrication of leaded electronic devices having a 20 mil (0.020 inch) pitch between leads. It is desirable to secure these devices to a PWB (Printed Wiring Board) using soldering techniques. For an electronic device having a 20 mil pitch, the device leads are approximately 14 mils (0.014 inches) wide and the distance between adjacent leads is about 6 mils (0.006 inches). This necessitates, for compatability with the device, closely-spaced circuit traces and closely-spaced solder pads having widths and pitches of similar magnitudes. However the device leads typically are smaller than the circuit traces to which the leads are to be connected and as a consequence an amount of solder exceeding the circuit trace thickness must be added to the PWB surface at the circuit trace connection point. Depositing and maintaining this added amount of solder, as will be discussed, presents problems with solder migration and with excess solder upon circuit traces.
FIG. 1 shows an electronic device 10, such as a microprocessor or electronic chip, having a plurality of leads 12. The electronic device 10 is mounted to a PWB (printed wiring board) 14 and the leads 12 are electrically connected to the PWB 14 at a plurality of corresponding solder pads 16. Connected to each solder pad 16 is a circuit trace 18. A circuit trace 18 may be electrically connected to an electrical via 20 which penetrates the PWB to electrically connect at another location, or a circuit trace 22 may electrically connect at a location on the same surface of the PWB 14.
FIG. 2 shows a cross-section of the electronic device 10 shown in FIG. 1. Note the item numbers are the same as those found in the discussion of FIG. 1. It is seen from FIG. 2 that the circuit trace 18 extends past the solder pad 16. For future discussion an area 21 is encircled.
Note the relative thickness of the solder pad 16 to the circuit trace 18. In order to secure the electronic device lead 12 to the solder pad 16, it is necessary to heat the region surrounding the solder pad 16 to a temperature above the reflow temperature of the solder. Typically the entire PWB 14 is heated to a temperature above this reflow temperature. While this is required to reflow the solder on the solder pad, if the material of the solder pad 16 and the circuit trace 18 are solderably compatible, that is, if the solder of the pad wets the circuit trace material, then at the solder reflow temperature the solder pad 16 loses its configuration as a portion of the solder migrates along the circuit trace 18 as shown in FIG. 3 such that there may not be adequate solder to secure the lead.
If the circuit trace 18 was thick enough such that it extended to the height at which solder pad 16 extends as shown in FIG. 2, then upon reflow the configuration of the circuit trace and the area of the solder pad would not change. Under these circumstances both the circuit trace 18 and the solder pad 16 could be solderably compatible. However, because the leads from electronic devices are typically smaller than the circuit traces, in order to secure a relatively small lead from the electronic device to a relatively large circuit trace on a PWB, a minimum amount of solder is required. Typically this minimum amount of solder must be of a thickness much greater than the thickness of a typical circuit trace. For this reason in order to maintain the flexibility to design thin circuit traces and provide a thick solder deposition in the area to which a lead is to be secured, another approach must be found for the deposition of solder on a PWB for securing the lead of an electronic component.
A second problem exists when the copper circuit traces are covered with a shielding material, such as tin-lead solder, to prevent oxidation of the circuit traces, which are typically made of copper. Although not shown in FIG. 1, after the electronic component 10 is in place on the PWB, a solder mask is placed over all of the areas on the PWB 14 in which solder should not reflow. Specifically, a solder mask is secured to the PWB to cover the plurality of circuit traces 18. When the PWB is heated so that the solder will reflow, if the circuit traces 18 are shielded with the same solder that is to be reflowed, then this shielding solder over the traces also may reflow. Since the solder mask is secured to the PWB, there is pressure downward compressing the circuit traces and solder shielding such that upon reflow, the solder shielding is compressed and loses shape as the solder deposits over the circuit traces begin to flatten and widen across the PWB. As mentioned before, the spacing between the circuit traces may be fairly small. The shielding solder over the closely spaced circuit traces 18 may be sufficiently deformed such that adjacent circuit traces electrically contact one another and form an electrical short, which is clearly unacceptable.
It is therefore an object of this invention to provide a solder deposition method and a circuit trace/solder pad interface that does not permit solder on a given solder pad to migrate along the associated circuit trace.
It is another object of this invention to provide a solder deposition method and a circuit trace/solder pad interface where the circuit traces do not reflow and lose their configuration such that short circuits with adjacent circuit traces are possible.
It is still another object of this invention to provide a method and a circuit trace/solder pad interface so that adequate solder may be deposited upon solder pads which are electrically connected to adjacent circuit traces such that after solder reflow there will be adequate amounts of solder to ensure that the leads of electronic components will be properly bonded at the solder pad.